1. Field of the Invention
The present invention relates to a vertical alignment liquid crystal display device in which liquid crystal molecules are aligned in a direction approximately perpendicular to a panel plane when a voltage is not applied. In particular, the present invention relates to a liquid crystal display device in which a retardation film is placed between a liquid crystal panel and a polarizing plate.
2. Description of the Prior Art
Liquid crystal display devices have the advantages that they are thin and light compared to cathode-ray tube (CRT) displays and that they can be driven at low voltages and have low power consumption. Accordingly, liquid crystal display devices are used in various kinds of electronic devices including televisions, notebook personal computers (PCs), desktop PCs, personal digital assistants (PDAs), mobile phones, and the like. In particular, active matrix liquid crystal display devices, in which a thin film transistor (TFT) is provided as a switching element for each picture element (sub-pixel), show excellent display characteristics comparable to those of CRT displays because of the high driving capabilities thereof, and therefore have come to be widely used even in fields in which CRT displays have been heretofore used, such as desktop PCs and televisions.
In general, a liquid crystal display device includes two transparent substrates and liquid crystals contained between these substrates. On one substrate, a picture element electrode, a TFT, and the like are formed for each picture element. On the other substrate, color filters facing the picture element electrodes, and a common electrode common to the picture elements, are formed. Hereinafter, the substrate on which the picture element electrodes and the TFTs are formed is referred to as a TFT substrate, and the substrate placed to face the TFT substrate is referred to as a counter substrate. Further, the structure made by filling the liquid crystals into the space between the TFT substrate and the counter substrate is referred to as a liquid crystal panel.
Heretofore, twisted nematic (TN) liquid crystal display devices have been popular in which horizontal alignment-type liquid crystals (liquid crystals with positive dielectric anisotropy) are contained between a TFT substrate and a counter substrate and in which liquid crystal molecules are aligned in a twisted manner. However, the TN liquid crystal display devices have the disadvantage that viewing angle characteristics are poor and that contrast and color greatly change when the screen is viewed from an oblique direction. Accordingly, in recent years, vertical alignment (VA) liquid crystal display devices, which are excellent in viewing angle characteristics compared to TN liquid crystal display devices, have come to be widely used. In VA liquid crystal display devices, vertical alignment-type liquid crystals (liquid crystals with negative dielectric anisotropy) are contained between a TFT substrate and a counter substrate.
FIG. 1 is a schematic diagram showing the constitution of a known VA liquid crystal display device. A liquid crystal panel 10 is constituted by filling vertical alignment-type liquid crystals into the space between a TFT substrate and a counter substrate as described above. On one side (lower side in the drawing) of the liquid crystal panel 10, a polarizing plate 11 is placed. On the other side (upper side in the drawing), a polarizing plate 12 is placed. The polarizing plate 11 has a structure in which a polarizing layer 11a is sandwiched between protective layers 11b and 11c. Similarly, the polarizing plate 12 has a structure in which a polarizing plate 12a is sandwiched between protective layers 12b and 12c. These polarizing plates 11 and 12 are placed with the absorption axes thereof oriented at right angles to each other.
A retardation film 13 having a negative retardation is placed between the liquid crystal panel 10 and the polarizing plate 11, and a retardation film 14 having a positive retardation is placed between the liquid crystal panel 10 and the polarizing plate 12.
As shown in FIG. 2A, the retardation film 13 has a negative retardation in the direction perpendicular to the film plane. When the refractive indices in the X, Y, and Z directions are respectively denoted by nx, ny, and nz, nx=ny>>nz is satisfied. This film 13 having a negative retardation is referred to as a c plate.
Meanwhile, as shown in FIG. 2B, the retardation film 14 has a positive retardation in an in-plane direction of the film plane. When the refractive indices in the X, Y, and Z directions are respectively denoted by nx, ny, and nz, nx>>ny=nz is satisfied. This film 14 having a positive retardation is referred to as an a plate. Heretofore, norbornene resin films, for example, films obtained by stretching ARTON films, manufactured by JSR Corporation, have been used as these retardation films 13 and 14. The retardations of these norbornene resin films have little wavelength dependence, and characteristics thereof are flat.
Viewing angle characteristics when these retardation films 13 and 14 are used are shown in FIG. 3. Note that FIG. 3 shows the results of calculating light transmittance in black display when the screen is viewed at the orientation of 45° (φ=45°) with respect to the horizontal direction (X direction) of the liquid crystal panel 10 and from the direction of 60° (θ=60°) with respect to the normal to the liquid crystal panel 10 as shown in FIG. 4. From this FIG. 3, it can be seen that, in the case where the retardation films 13 and 14 are used, leakage light in oblique directions is significantly reduced compared to the case where there are no retardation films.
In Japanese Unexamined Patent Publication No. 2003-279992, a technology is described in which, in a homogeneous alignment liquid crystal display device, a TN liquid crystal display device, or an STN liquid crystal display device using horizontal alignment-type liquid crystals, a retardation layer containing norbornene or polysulfane as a main constituent is placed between a liquid crystal panel and an optical film for viewing angle compensation on one side of the liquid crystal panel to suppress coloring due to the wavelength dependence of retardation.
Moreover, in Japanese Unexamined Patent Publication No. Hei 7(1995)-198943, a liquid crystal display device is described in which a retardation film and an elliptically polarizing plate are placed outside a liquid crystal panel (liquid crystal cell) to improve viewing angle characteristics.